Researchers have long suspected that tsunamis sometimes merge to create a single more powerful wave, but the tsunami caused by the earthquake that rocked Japan earlier this year was the first time they actually saw it happen.

The tsunami that hit Japan surged miles inland along the coast.

The biggest earthquake ever recorded was in 1960, in Chile. The 9.5 quake killed and injured thousands there, and triggered a deadly tsunami that hit Hawaii, the Philippines, and Japan.

“It was a mystery for a long time,” said Tony Song, a research scientist at NASA’s Jet Propulsion Laboratory, of the resulting tsunami that crossed the Pacific. “How it happened so far away,” and caused so much damage.

The explanation, it turned out, was that it was a merging tsunami. When the quake hit, a wave radiated out from the epicenter. But it didn’t form a neat circle, like when you drop a stone into a pond. Instead, underwater mountains and ridges broke the wave into segments that headed in different directions. Then that mountainous underwater topography guided some of those sections back together. They merged into a single double-size wave that could cross the ocean without losing much steam.

The explanation made sense, but Song and other researchers had never seen it actually happen until the tsunami in March.

“We knew waves could merge, but had never seen evidence before,” said Song. This animation from NASA shows it happening. The wave radiates out from the earthquake’s epicenter. Then the animation zooms in to show the waves, which have been broken up, merging.

Scientists at NASA and Ohio State University lucked into the find. Three satellites capable of measuring sea level changes down to a matter of centimeters happened to be in the area. When they passed over the tsunami, two of them measured one height for the wave. A third, the NASA-Centre National d’Etudes Spaciales Jason-1 satellite, measured a different height, twice what the other satellites saw. It was a merging tsunami. The wave front the satellites measured was the one heading east from the earthquake’s epicenter, not the one that ravaged the Japanese coast. But the discovery at least raises the question of whether or not the wave going in the other direction, the one that did hit Japan, was also a merging tsunami. Song says, unfortunately, he doesn’t have the data to say one way or the other.

“Specifically, our study is about the double power away from Japan, instead of toward Japan, though the same mechanism might work in both directions.”

Merging tsunamis occasionally hit Crescent City, California. According to Song, the topography of the ocean floor has the same effect when earthquakes near Alaska trigger tsunamis headed for California.

Now that he’s observed one of these waves in action, Song says scientists will be able to make better forecasts, predicting where tsunami waves are likely to merge, and where they’re likely to hit land.

A couple of other items you may have missed over the last few months about the tsunami:

Song and his colleagues presented their analysis at the San Francisco meeting of the American Geophysical Union this week (#AGU11).

UPDATED and CORRECTED: An earlier version of this post may have misled some readers by implying that actual satellite data confirmed the merger of two tsunami waves into the front that hit Japan. The data was observed for waves moving in the opposite direction.

]]>http://blogs.kqed.org/climatewatch/2011/12/09/wave-that-hit-japan-was-a-merging-tsunami/feed/2How “Tsunami-Ready” Are You?http://blogs.kqed.org/climatewatch/2011/03/24/how-tsunami-ready-are-you/
http://blogs.kqed.org/climatewatch/2011/03/24/how-tsunami-ready-are-you/#commentsFri, 25 Mar 2011 03:53:29 +0000http://blogs.kqed.org/climatewatch/?p=11992Crescent City has the drill down

A sign along Highway 101 in Crescent City marks the tsunami hazard zone. Officials say they can evacuate the hazard zone in about two hours. (Photo: Craig Miller)

Of course, after facing down 34 tsunami events in the past 100 years, I suppose they should have it down. When I was there to cover the aftermath of the March 11 event that pretty much took out the working harbor, it was clear that the possibility of a seismically-triggered surge is never far from the public consciousness in Del Norte County.

“It’s here with us from the names of buildings to the names of the businesses,” Cindy Henderson told me. “Tsunami is our world. So yeah, it is a very big threat,” said Henderson, who heads emergency services for the county. “We do have others we have to prepare for but in the backs of our minds, we are always thinking about tsunamis — every time there’s a big earthquake.”

Which isn’t to say that the northwest corner of California has a lock on tsunami preparedness. Crescent City is one of 17 California communities that has earned the “Tsunami-Ready” designation by NOAA. You can check this map to see if your town is one of them.

My companion radio feature for The California Report (Friday morning, 3/25) profiles some ways in which Crescent City has prepared.

]]>http://blogs.kqed.org/climatewatch/2011/03/24/how-tsunami-ready-are-you/feed/0Crescent City: “It’s a mess, all right.”http://blogs.kqed.org/climatewatch/2011/03/13/crescent-city-its-a-mess-all-right/
http://blogs.kqed.org/climatewatch/2011/03/13/crescent-city-its-a-mess-all-right/#commentsMon, 14 Mar 2011 00:47:30 +0000http://blogs.kqed.org/climatewatch/?p=11712Astoundingly, the mess left by Friday’s tsunami was confined to the harbor…for now. No, it wasn’t 1964, when a tsunami triggered by a quake off Alaska took out the harbor and half the town, killing 11 people. But hanging out there on the northwestern tip of California, Crescent City is “Tsunami Central” for this part of the continent.

Through a quirk of geography, the opening to the city’s horseshoe-shaped inner harbor seems to be lined up perfectly for seismically induced waves sweeping across the Pacific. The most impressive video clips I’ve seen were shot by Bryant Anderson. Both are posted on the website of the Crescent City Daily Triplicate. Anderson captured one clip while the tsunami sirens were still sounding:

Friday’s surge, which peaked with an 8-foot wave, had its way with what local officials say had been the most productive seafood landing harbor on the California coast. The next challenge is environmental, as salvage crews wait out the weekend’s extreme weather to tackle the potential threat of nearly 60 damaged and sunken boats, many full of diesel fuel, industrial lubricants and other contaminants. “Every one of the vessels has fuel on board,” said Alexia Retallack, who is helping coordinate recovery efforts for the state Department of Fish & Game’s Office of Spill Prevention & Response. “So every vessel that’s either sunken or currently in the harbor and compromised is a source of petroleum product,” she said.

They take the tsunami threat seriously around here. The area has a well-rehearsed evacuation plan. One resident told me a drill had been planned for mid-April. So when she was awakened by sirens early Friday morning, she first thought she had the date confused. Even the little area map in the hotel’s “guest services” book features a bold arc, clearly identifying the evacuation zone. Most hotels are inside of it, so they were evacuated with everyone else near the shoreline last week. Here are some photos of the scene I encountered when I arrived on Saturday:

Commercial fishing boats piled up like tub toys in one corner of the basin. (Photo: Craig Miller)

Nearly the entire system of floating docks was torn out by the tsunami. (Photo: Craig Miller)

Crescent City's harbor has proven to be a magnet for Pacific tsunamis, including a devasting wave in 1964 and one that caused less damage in 2006. (Photo: Craig Miller)

Deckhand Ivan Simpson works on the 48' FV Rogue, which now has no home port. Boats also lost a large amount of crabbing gear that was in the ocean. Simpson was the source of my headline quote. (Photo: Craig Miller)

There are more video clips of the Crescent City surge posted on YouTube. This one, while nothing like the horrifying pictures we’ve seen from Japan, shows the surge roiling at the mouth of the inner harbor “like inside a washing machine,” according to one official. Witnesses say the outgoing surge before the approaching wave drained the entire inner harbor, leaving fishing boats and docks sitting on a vast mud flat…until the water returned with unforgiving force.

Harbor Master Rich Young says he’s hoping to have at least a semblance of a home port restored for the fishing fleet by November, in time for the next crab season. You can hear my companion radio story, archived on the California Report website.